Cool Diffusion Flame Research Articles

Cool diffusion flames in a stably stratified stagnation flow

Cool diffusion flames have been a growing research topic since their discovery in 2012. Until now their study has been hindered by the high cost of the experimental systems used to observe them. A method is presented here for observing cool diffusion flames inexpensively using a pool of liquid n-heptane and parallel plates heated so as to produce a stably stratified stagnation flow. The flames were imaged with a color camera and an intensified camera. Measurements included gas phase temperatures, fuel evaporation rates, and formaldehyde yields. These are the first observations of cool flames burning near the surfaces of fuel pools. The measured peak temperatures were between 705 and 760 K and were 70 K above the temperature of the surrounding air. Autoignition first occurred at 550 K.

Effects of Carbon Chain Length on N-Alkane Counterflow Cool Flames: A Kinetic Analysis

An in-depth understanding of the low-temperature reactivity of hydrocarbon fuels is of practical relevance to developing advanced low-temperature combustion techniques. The present study aims to study the low-temperature chemistry of several large n-alkanes with different carbon chain lengths in counterflow cool diffusion flames by kinetic analysis. The large n-alkanes that were chosen are n-heptane (NC7H16), n-decane (NC10H22) and n-dodecane (NC12H26), which are important components of practical fuels. Firstly, the thermochemical structure of a typical cool diffusion flame is understood through its comparison with that of a hot diffusion flame. The boundary conditions, including the ozone concentration, fuel concentration and flow velocity—where cool flames can be established—are identified with a detailed chemical mechanism that evaluates the low-temperature reactivity of the investigated n-alkanes. The results show that the n-alkane with a longer carbon chain length is more reactive than the smaller one, thereby indicating the order of NC12H26 > NC10H22 > NC7H16. This trend is qualitatively similar to the findings from non-flame reactors. The reaction pathway and sensitivity analysis are performed to understand the effects of carbon chain length on the low-temperature reactivity. The contribution of an n-alkane with a longer carbon chain to the dehydrogenation reaction, oxidation reaction and isomerization reaction is greater than that of a smaller n-alkane, and abundant O and OH radicals are generated to promote the fuel low-temperature oxidation process, thereby resulting in an enhanced low-temperature reactivity. The effects of ozone addition on the low-temperature reactivity of n-alkanes are also highlighted. It is found that the addition of ozone could provide a large number of active O radicals, which dehydrogenate with the fuels to generate OH radicals and then promote fuel low-temperature oxidation. The present results are expected to enrich the understanding of the low-temperature characteristics of large n-alkanes.

Experimental and kinetic studies of extinction limits of counterflow cool and hot diffusion flames of ammonia/n-dodecane

The present study focuses on the effects of ammonia on the extinction limits of n-dodecane cool and hot diffusion flames and the kinetic interactions between ammonia and n-dodecane at low and high temperatures. The extinction limits of cool and hot diffusion flames for ammonia/n-dodecane blends are measured in an atmospheric counterflow burner. The results show that the extinction limits of both cool and hot flames decrease with the increase of the ammonia blend ratios and an obvious non-linear tendency can be found for the hot flames. Moreover, the laminar flame speeds and ignition delay times of ammonia/n-dodecane blends are also measured and a detailed ammonia/n-dodecane model is developed and validated with the measured data. With the assistance of kinetic modeling, the physical and chemical effects of ammonia on the extinction limits of ammonia/n-dodecane hot and cool diffusion flames are examined. It is found that the physical effects of ammonia suppress the extinction limits of both cool and hot diffusion flames, relative to the nitrogen dilution. Chemical effects of ammonia play a leading role and increase the extinction limits of hot diffusion flames but decrease the extinction limits of cool diffusion flames, indicating that the ammonia promotes the high-temperature oxidation of n-dodecane but inhibits the low-temperature oxidation of n-dodecane. Kinetic analyses reveal that ammonia addition inhibits the low-temperature chemistry of n-dodecane via NH3 + OH <=> NH2 + H2O (NH3 directly involves the reactions slowing down the n-dodecane H-abstraction reaction) and NO + RO2 <=> RO + NO2 (NO produced from NH3 and NO competes with low-temperature chain-branching reaction pathways for RO2). Moreover, different major reaction pathways of ammonia oxidation at low and high temperature are summarized: NH3 → NH2 → H2NO → HNO → NO is responsible for the ammonia consumption at low temperature, while NH3 → NH2 → NH → N → NO is the one at high temperature.

Spherical gas-fueled cool diffusion flames

An improved understanding of cool diffusion flames could lead to improved engines. These flames are investigated here using a spherical porous burner with gaseous fuels in the microgravity environment of the International Space Station. Normal and inverse flames burning ethane, propane, and n-butane were explored with various fuel and oxygen concentrations, pressures, and flow rates. The diagnostics included an intensified video camera, radiometers, and thermocouples. Spherical cool diffusion flames burning gases were observed for the first time. However, these cool flames were not readily produced and were only obtained for normal n-butane flames at 2 bar with an ambient oxygen mole fraction of 0.39. The hot flames that spawned the cool flames were 2.6 times as large. An analytical model is presented that combines previous models for steady droplet burning and the partial-burning regime for cool diffusion flames. The results identify the importance of burner temperature on the behavior of these cool flames. They also indicate that the observed cool flames reside in rich regions near a mixture fraction of 0.53.

A numerical study on near-limit extinction dynamics of dimethyl ether spherical diffusion flame

The near-extinction oscillatory dynamics and underlying mechanism of the dimethyl ether (DME) spherical diffusion flames in the hot- and cool-flame conditions were studied by the numerical approach with detailed chemistry and transport models. It was found that the self-sustaining spherical cool diffusion flame could be readily obtained in a wide range of parameters, and additionally, the flammability limit could be considerably extended by the cool-flame chemistry. Strong oscillations that can trigger flame extinguishment prior to the steady-state extinction turning point were observed in either the hot or cool flame regime. The DME hot flame near extinction was governed by a single oscillatory mode with a fixed frequency (1 Hz) that was irrelevant with the ambient oxygen level. By contrast, the cool-flame extinction was controlled by dual oscillatory modes which had rather distinct frequencies, and the oscillatory period of the high-frequency mode increased significantly when approaching the extinguishment. Additionally, the transient dynamics of cool flame near extinction was much more complicated, due to the existence of strong coupling between the high-frequency and low-frequency oscillatory modes which was affected by the phase difference. The governing reactions that controlled the oscillatory extinction in hot- and cool-flame regions were revealed using a logarithmic sensitivity index. It was found that the hot-flame oscillatory extinction was controlled by the competition of high-temperature exothermic/endothermic reactions with the chain branching/termination reactions involving small molecules. The cool-flame oscillation was controlled by the low-temperature branching and termination competition in the negative temperature coefficient regime.

Dynamics and burning limits of near-limit hot, warm, and cool diffusion flames of dimethyl ether at elevated pressures

The near-limit diffusion flame regimes and extinction limits of dimethyl ether at elevated pressures and temperatures are examined numerically in the counterflow geometry with and without radiation at different oxygen concentrations. It is found that there are three different flame regimes—hot flame, warm flame, and cool flame—which exist, respectively, at high, intermediate, and low temperatures. Furthermore, they are governed by three distinct chain-branching reaction pathways. The results demonstrate that the warm flame has a double reaction zone structure and plays a critical role in the transition between cool and hot flames. It is also shown that the cool flame can be formed in several different ways: by either radiative extinction or stretch extinction of a hot flame or by stretch extinction of a warm flame. A warm flame can also be formed by radiative extinction of a hot flame or ignition of a cool flame. A general €-shaped flammability diagram showing the burning limits of all three flame regimes at different oxygen mole fractions is obtained. The results show that thermal radiation, reactant concentration, temperature, and pressure all have significant impacts on the flammable regions of the three flame regimes. Increases in oxidizer temperature, oxygen concentration, and pressure shift the cool flame regime to higher stretch rates and cause the warm flame to have two extinction limits. At elevated temperatures, it is found that there is a direct transition between the hot flame and warm flame at low stretch rates. The results also show that, unlike the hot flame, the cool flame structure cannot be scaled by using pressure-weighted stretch rates due to the its significant reactant leakage and strong dependence of reactivity on pressure. The present results advance the understanding of near-limit flame dynamics and provide guidance for experimental observation of different flame regimes.

Cool diffusion flames of butane isomers activated by ozone in the counterflow

Ignition in low temperature combustion engines is governed by a coupling between low-temperature oxidation kinetics and diffusive transport. Therefore, a detailed understanding of the coupled effects of heat release, low-temperature oxidation chemistry, and molecular transport in cool flames is imperative to the advancement of new combustion concepts. This study provides an understanding of the low temperature cool flame behavior of butane isomers in the counterflow configuration through the addition of ozone. The initiation and extinction limits of butane isomers’ cool flames have been investigated under a variety of strain rates. Results revealed that, with ozone addition, establishment of butane cool diffusion flames was successful at low and moderate strain rates. iso-Butane has lower reactivity than n-butane, as shown by higher fuel mole fractions needed for cool flame initiation and lower extinction strain rate limits. Ozone addition showed a significant influence on the initiation and sustenance of cool diffusion flames; as ozone-less cool diffusion flame of butane isomers could not be established even at high fuel mole fractions. The structure of a stable n-butane cool diffusion flame was qualitatively examined using a time of flight mass spectrometer. Numerical simulations were performed using a detailed chemical kinetic model and molecular transport to simulate the extinction limits of the cool diffusion flames of the tested fuels. The model qualitatively captured experimental trends for both fuels and ozone levels, but over-predicted extinction limits of the flames. Reactions involving low-temperature species predominantly govern extinction limits of cool flames. The simulations were used to understand the effects of methyl branching on the behavior of n-butane and iso-butane cool diffusion flames.

Study of the low-temperature reactivity of large n-alkanes through cool diffusion flame extinction

The low-temperature oxidation of hydrocarbon fuels has received increasing attention as advanced engines seek to operate in less conventional combustion regimes. Large n-alkanes are a notable component of many real transportation fuels and possess strong reactivity in this important low-temperature range. These n-alkanes have been studied extensively in various canonical kinetic experiments but seldom in systems with strong coupling between low-temperature chemistry, transport, and heat release. To address this issue, the present study investigates self-sustaining n-alkane cool diffusion flames in a counterflow burner. The extinction limits of both hot diffusion flames and cool diffusion flames are measured at atmospheric pressure for a range of n-alkanes from n-heptane to n-tetradecane. It is observed that while these fuels behave similarly for hot flames, the larger n-alkanes are substantially more reactive in the low-temperature cool flame regime. Moreover, ozone addition strongly enhances the low-temperature chemistry to the point where the differences in fuel reactivity are nearly suppressed.The experimental measurements are compared with numerical simulations employing both detailed and reduced chemical kinetic models of various sizes. Although the different kinetic models adequately predict the extinction limits of the hot flames, a large scatter is present in the model results for cool flames, and a general overprediction of the measured cool flame extinction limit is observed for all of the fuels studied. This implies that the cool flame heat release is not well agreed upon by the current chemical kinetic models, despite their capability to reproduce many homogeneous reactor experiments at low temperatures. Furthermore, it is observed that the cool flame heat release is spread over a substantial number of reactions involving large molecules, a trait that makes it particularly difficult to create reduced kinetic models that can accurately describe cool flame behavior. The results of this study suggest that the cool flame platform can provide crucial validation of the coupling between chemistry, transport, and heat release in flames at low temperatures.

Flame structure and ignition limit of partially premixed cool flames in a counterflow burner

Due to their natural coupling of low-temperature chemistry and transport, cool flames are a valuable platform for drawing fundamental understandings of complicated phenomena relevant to real engines. In this study, self-sustaining partially premixed cool flames of dimethyl ether are investigated in detail through the use of an ozone-assisted counterflow burner. A double cool flame with distinct diffusion flame and premixed flames sides is visibly observed at increased fuel loading and equivalence ratio. The examination of double cool flames and double hot flames through planar laser-induced fluorescence measurements reveals the stark differences in the role of CH2O in each. The results show that while CH2O is one of the main product species of the cool flame via low-temperature chemistry, for the hot flame it is only a short-lived intermediate produced in the preheat zone. Comparisons of experimental results with numerical calculations based upon detailed chemical kinetic models show fair agreement on the double cool flame structure but a noticeable discrepancy in the prediction of the second-stage ignition limit, which triggers the transition from cool flames to hot flames. The critical strain rate for second-stage ignition is shown to be much more sensitive to fuel addition on the premixed side of the double flame than on the diffusion side. A mechanism for second-stage ignition in partially premixed cool flames is proposed based upon numerical modeling and experimental observations: H2O2 is primarily formed in the premixed cool flame, diffuses toward the stagnation plane, and then finally decomposes into OH radicals upon approaching the cool diffusion flame.

Formation of a cool diffusion flame and its characteristics

Low-temperature (∼700 K) “cool” flames formed with n-heptane fuel were observed in the experiments conducted under microgravity. Recently, Won et al. demonstrated experimentally that such “cool” flames could be established under normal gravity. However, as ozone was added to air for supporting the flames in that experiment, the fundamental question on the formation of a self-supported n-heptane/air “cool” flame remained unanswered. A numerical investigation is conducted for (1) finding an answer to this question and (2) determining a procedure for establishing a “cool” flame. A standard opposing-jet burner is considered. Investigations are performed using a well-tested CFD code. A reduced mechanism that successfully captured the “cool” flames in microgravity is used. Calculations have demonstrated that a “cool” diffusion flame can be established with n-heptane under normal gravity without resorting to flame-speed promoters such as ozone, pressure or temperature. Using velocities of 4 and 7 cm/s and temperatures of 400 and 300 K for the fuel and air jets, respectively, a stable “cool” flame was obtained. Flame was ignited through increasing the temperature of the air jet to a value >724 K and then a self-sustained “cool” flame was obtained by decreasing the temperature back to 300 K. Parametric studies are performed on the applied gravitational force such that the heavy n-heptane flowed upward from the bottom nozzle or downward from the top nozzle. In general, flame remained flat with or without the gravity. However, when the fuel jet was at the bottom, it expanded faster and moved the flame closer to the fuel nozzle. “Cool” flame could not be stabilized for the same velocity and temperature conditions when fuel flowed from the top. “Cool” and normal diffusion flames obtained with identical flow conditions are compared. Limiting stretch rate for extinguishing the “cool” flame is determined.

Thermo-kinetic dynamics of near-limit cool diffusion flames

The dynamics of near-limit cool diffusion flames are investigated experimentally and numerically by studying transient flame evolution and instability. In order to observe the effects of chemistry-transport coupling on ignition and instability in a cool diffusion flame, diffusion flames of n-heptane and pure oxygen are sensitized by ozone in a counterflow burner. First, it is found from experimental observations that the immediate addition of ozone to the oxidizer side of a frozen flow either initiates a cool diffusion flame or triggers a two-stage ignition into a hot flame. Second, a cool flame near its extinction limit can be destabilized by a perturbation of the fuel mole fraction, eventually leading to flame extinction. The recorded flame chemiluminescence signals reveal repeated flame oscillations before extinction. Next, details of the transient dynamics of near-limit cool diffusion flames are explored through numerical calculations by adopting the same perturbation method. Experimental observations of unsteady flame initiation and instability are simulated, and it is found that the ignition process is highly sensitive to the perturbed ozone mole fraction and that the ignition delay times in a counterflow configuration are significantly affected by the diffusive transport of species with high concentration. The instability mechanism of a cool flame is found to be distinct from that of diffusive-thermal instability. The results show that the instability behavior of a cool flame is a thermo-kinetic instability, which is triggered and controlled by the chemical kinetics associated with the OH radical population in the negative temperature coefficient chemical kinetic regime coupled with heat production and loss.

Self-sustaining n-heptane cool diffusion flames activated by ozone

A novel method to establish self-sustaining cool diffusion flames with well-defined boundary conditions is experimentally demonstrated by adding ozone to the oxidizer stream in counterflow configuration. It is found that the atomic oxygen produced through the decomposition of ozone dramatically shortens the induction timescale of the low temperature chemistry, extending the flammable region of cool flames. Thus, it enables the establishment of self-sustaining cool flames at the pressure and timescales at which normal cool flames may not be observable. The present method, for the first time, provides an opportunity to study cool flame dynamics, structure, and chemistry simultaneously in well-known flame geometry. Extinction limits of n-heptane/oxygen cool diffusion flames are measured and a cool diffusion flame diagram is experimentally determined. Numerical simulations reveal that the extinction limits of cool diffusion flames are strongly governed by species transport and low temperature chemistry activated by ozone decomposition. The structure of cool diffusion flame is further investigated by measuring the temperature and species distributions with a micro-probe sampling technique. The kinetic model over-predicts the rate of n-heptane oxidation, the heat release rate, and the flame temperature. Measurements of intermediate species, such as CH2O, acetaldehyde, C2H4, and CH4, suggest that the model over-predicts the QOOH thermal decomposition reactions to form olefins, resulting in substantial over-estimation of C2H4, and CH4 concentrations. The new method and data of the present study will contribute to promote understandings of cool flame chemistry.

Determination of arsenobetaine, arsenocholine, and tetramethylarsonium cations by HPLC thermochemical hydride generation-atomic absorption spectrometry

A novel high-performance liquid chromatography-atomic absorption spectrometry (HPLC-AAS) interface based on thermochemical hydride generation has been developed and optimized for the determination of arsenobetaine [(CH3)3As+CH2COOH], arsenocholine [(CH3)3As+CH2CH2OH], and tetramethylarsonium [(CH3)4As+] cations. In this quartz interface, the methanolic HPLC eluent was nebulized by thermospray effect and pyrolyzed in a methanol/oxygen kinetic flame and the analytes were thermochemically derivatized to the hydride derivative in the presence of excess hydrogen. The volatile derivative was then transported to a cool diffusion H2/O2 flame atomizer. The fact that arsenic pentoxide was also derivatized, and that no signal was observed in the absence of postthermospray hydrogen or the absence of cool diffusion flame, corroborated the thermochemically mediated arsine-generation mechanism. Factorial models predicting the performance of the interface at different levels of five selected variables suggested that (a) both reverse- and normal-phase HPLC eluents were compatible with the interface and (b) the performance of this system was relatively insensitive (less than 50% variation in response) to changes over wide ranges in the operating parameters. At concentrations up to 10-fold excess, potential interferents [(CH3)3S+, (CH3)3Pb+] did not affect, significantly, the postulated thermochemical hydride generation (THG) process. However, a 10-fold molar excess of (CH3)3Se+ decreased the response of the analyte by 48%. Virtually identical responses were observed for equimolar amounts of tetramethylarsonium, arsenocholine, arsenobetaine [As(-III)], and dimethylarsinic acid [As(III)]. Arsenic pentoxide [As(V)] was detected with 75% efficiency, relative to the response of the organoarsenic compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies in atomic-fluorescence spectroscopy—VI : The fluorescence characteristics and analytical determination of bismuth with an iodine electrodeless discharge tube as source

The atomic-fluorescence characteristics of bismuth atoms in cool nitrogen-hydrogen and argon-hydrogen diffusion flames burning in air are described. Excitation is obtained from the non-resonance iodine line at 2061.63 Å emitted by a microwave-excited electrodeless discharge tube operating at 2450 Mc/s. Fluorescence of the bismuth resonance line at 2061.70 Å is observed and also direct-line fluorescence at 2697 and 3025 A. In addition thirteen other much weaker lines were observed and two unidentified lines at 2880 and 2680 Å. The emissions at 2628 and 2938 Å appear to arise from “thermally assisted direct line fluorescence”. The most intense line at 3025 Å permits linear-dependence analytical atomic-fluorescence measurements to be made in the range 0.1–200 ppm with a detection limit of 0.05 ppm and with no problems of source scatter. No interference was observed from hundred-fold concentrations of fourteen ions. Matrix effects from aluminium and magnesium were overcome by raising the temperature of the cool diffusion flames. A bismuth microwave-excited electrodeless discharge tube was used as a source for atomic-absorption measurement in air-hydrogen and air-propane flames at 2231Å with a detection limit of 1 ppm and a linear-dependence analytical range of 10–100 ppm. With the iodine microwave-excited electrodeless discharge tube the detection limit for atomic absorption was 10 ppm at 2062 Å.